An STI has been known as a technique for electrically isolating a device formed on a silicon substrate. The STI involves the sequential steps of etching a silicon substrate through a mask of, for example, a silicon nitride film to form a trench therein; filling the trench with an insulating film such as SiO2; and planarizing the substrate by chemical mechanical polishing (CMP) by way of using the mask (silicon nitride film) as a stopper.
However, recently, with the trend of requirement for high integration and high speed of LSI (large scale integrated circuit), a design rule of semiconductor devices forming the LSI is getting finer and, at the same time, economization of power consumption is increasingly required. In case of forming a fine trench in the STI, a shoulder portion of the trench (an edge portion of an upper sidewall in the trench) formed by an etching tends to have an acute angle. As a result, a leakage current between a gate electrode and an active area through the shoulder portion increases, thereby causing a problem that power consumption becomes increased.
Thus, there is proposed an etching method including a first step of introducing a gaseous mixture containing HBr and N2 serving as a processing gas and rounding off an upper sidewall of a trench by plasma processing; a second step of making a trench in a silicon of the silicon substrate by plasma processing; and a third step of introducing a gaseous mixture containing HBr and Cl2 serving as a processing gas and rounding a bottom portion of the trench by plasma processing (see, e.g., Reference 1).
[Reference 1] Japanese Patent Laid-open Application No. 2003-218093
The etching method disclosed in Reference 1 is an excellent technique capable of forming a round shape (top rounding) in the shoulder portion of the trench by performing the first step to thereby reduce the leakage current. Since, however, it is a multi-step plasma etching method involving the first step of forming a shallow trench by plasma etching the silicon substrate, the second step and the third step, a shoulder of a mask may be damaged due to a prolonged etching time and, thus, a further improvement is required to enhance an accuracy of microprocessing. Further, in the method disclosed in Reference 1, since a temperature of the substrate (a temperature of a lower electrode) needs to be changed in order to accurately round the shoulder portion in the first and the second step, a time for adjusting the temperature is needed and, thus, the time required for sequential steps tends to be longer. Accordingly, a process capable of further improving a throughput is requested.